A Macaque type D retrovirus was isolated from an explant of retroperitoneal fibromatosis (RF) in a rhesus monkey with simian immune deficiency syndrome (SAIDS) characterized by persistent diarrhea, weight loss, lymphocytopenia, opportunistic infections and RF. To examine viral pathogenicity, four juvenile colony-born and four feral young adult Macaca nemestrina were inoculated with a 500X concentrate of filtered culture medium of dog thymus cells infected with Macaque type D/Washington retrovirus. At 5 weeks, one colony-born juvenile macaque died; it had generalized lymphoid depletion but only suggestive RF. At 10 weeks, retroperitoneal nodules were palpated in a colony-born juvenile macaque that at laparotomy at 18 weeks were histologically diagnosed as RF. After 13 months, the macaque with RF and the one dying at 5 weeks are the only animals with viremia as determined by isolation of the Macaque type D/Washington virus from blood plasma, and are the only animals without seroconversion following virus inoculation. Eight matched control M. nemestrina, inoculated with filtered culture medium from uninfected dog thymus cells, have remained healthy. To further examine the interplay between immunization and/or pathogenicity of the Macaque type D/Washington retrovirus isolate, a second viral transmission experiment is underway to achieve a higher incidence of disease using three age groups of M. nemestrina inoculated IV with unfrozen Macaque type D/Washington retrovirus isolated from an M. nemestrina with SAIDS. To date, several macaques have developed nodular abdominal lymphadenopathy, suggestive of retroperitoneal fibromatosis. However, no definite diagnostic, clinical or laboratory evidence of SAIDS has developed in these macaques, which have been continuously caged in a pathogen-free facility.